In recent years, the use of time division multipled digital switches in private branch exchange (PBX) telephone systems has become a virtual standard. In such systems, voice signals are digitized, and digital samples of the voice signals are switched throughout the system between various ports which normally constitute station lines of the PBX, interfaces to telephone company trunk lines, conference bridges, and the like. Similar digital switches are also being used as replacements for cross point switches in telephone central offices, and in toll switches within the toll telephone network.
As the cost of high speed digital electronic components is dropped, the cost of digital PBX equipment has also dropped and additional features have been added to private branch exchanges.
However, the digital switch still represents the single most expensive item within a private branch exchange and this has historically placed a lower limit on the number of PBX extension lines which may be economically used in a particular application. At some point, it makes more economic sense to revert to an older key-type telephone system with its clumsy, and increasingly expensive cabling requirements.
TDM digital switches in prior art PBXs have almost universally included a memory time switch within the digital signal path. Memory time switches are devices which temporarily store incoming digitized samples, bound for a particular destination and wait until the appropriate time slot on the appropriate data line or bus occurs, corresponding to the time slot/line combination for the recipient of the digitized sample, and then inserts the sample into the appropriate time slot on the appropriate line or bus. Such digital time switches are expensive, and have been a major factor in keeping the above-referenced lower limit on the practical number of PBX station lines relatively high.
Other prior art TDM switches have been constructed so that relatively complex reconfigurations of the system are necessary as the number of ports (which term includes trunk interfaces and PBX station lines) connected to the system increases.
Therefore, there is a need in the prior art to provide a TDM switch, particularly one suitable for use in a private branch exchange, which may be inexpensively constructed, particularly in applications in which a relatively small number of PBX station lines are to be connected. There is a further need to construct such a switch which allows a private branch exchange to expand gracefully, without reconfiguration, up to and past the point where the total number of ports exceeds the total number of transmission paths available through the switch.
In accomplishing the first objective, it is highly desirable to provide a TDM switch for private branch exchange which eliminates the need for a memory time switch in all applications in which the number of ports is less than or equal to the number of transmission paths, and which does not require such a memory time switch up to a system size in which the total number of ports is several times the total number of available transmission paths.
To accomplish the second object, it is highly desirable to provide a TDM switch which possesses an architecture which readily lends itself to the retrofitting of a memory time switch as the PBX system expands.
The present invention fulfills this need of the prior art by providing an improved time division multiplexed switch structure for which any port may be connected to any transmission path. Thus, any port may be readily connected by the system to any combination of time slot and transmission or highway line, the time slot/line combination comprising a transmission path through the switch.